1. Field of the Invention
The present invention relates generally to a data transmission apparatus and method in a mobile communication system, and in particular, to an apparatus and method for retransmitting data having a transmission error.
2. Description of the Related Art
Mobile communication systems chiefly use linear block codes such as convolutional codes stand turbo codes, for channel coding, Such a mobile communication system employs an HARQ (Hybrid Automatic Repeat (or Retransmission) reQuest) scheme, which requests retransmission of data packets upon detection of an FEC (Forward Error Correction) code and an error.
A mobile communication system employs the HARQ scheme to increase data transmission efficiency, i.e. throughput and improve system performance using a channel coding scheme. The mobile communication system generally refers to a satellite system, an ISDN (Integrated Services Digital Network) system, a digital cellular system, a CDMA-2000 (Code Division Multiple Access-2000) system, a UMTS (Universal Mobile Telecommunication System) system and an IMT-2000 (International Mobile Telecommunication-2000) system. Further, the convolutional codes and the turbo codes are used for the FEC code.
Operation of the general HARQ scheme will be described below with reference to FIGS. 1 to 3.
FIG. 1 illustrates a packet retransmission process in the general HARQ scheme. Referring to FIG. 1 a receiver RX receives initial packet data transmitted from a transmitter TX (Step 101). The receiver determines whether an error has occurred in the received initial packet data. Upon detecting an error from the initial packet data, the receiver transmits a retransmission request message NAK Negative Acknowledgement) with only packet ID (identification) information including a version number and a sequence number to the transmitter (Step 102). It is possible for the receiver to report information on the retransmission-requested packet data to the transmitter by transmitting the retransmission request message NAK with the packet H) information including a version number and a sequence number. Although now shown in FIG. 1, for the error-fee packet data received, the receiver transmits an ACK (Acknowledgement) signal with the packet ID information, Upon receipt of the retransmission request message NAK, the transmitter transmits the retransmission-requested packet data (Step 103) Upon receipt of the packet data retransmitted from the transmitter, the receiver determines again whether an error has been detected from the retransmitted packet data and upon detecting an error, repeats the above-stated process (Steps 104 and 105). However, if no error has been detected from the retransmitted packet data, the receiver transmits an ACK signal with the packet ID information including a version number and a sequence number to the transmitter (Step 106).
As described above, the conventional HARQ scheme repeats the packet retransmission process until the receiver transmits an ACK signal after successful decoding, or repeats the packet retransmission process predetermined times. Here, the “predetermined times” refers to a possible retransmission frequency of the same packet data, specified in the HARQ scheme. When the HARQ scheme continuously detects an error from the transmission data, i.e., when the channel environment is bad, a time period t1–t0 required in transmitting the same packet data is increased drastically decreasing the overall troughput. Although FIG. 1 shows only the process for retransmitting the packet data having a transmission error, the HARQ scheme actually operates in a Selective-Repeat ARQ mode, so that the transmitter continuously transmits the packet data no matter whether the packet data has a transmission error. Therefore, upon receipt of the packet ID information, i.e., a version number and a sequence number, of the erroneous packet data from the receiver, the transmitter repeats the process for retransmitting only the packet data having a transmission error.
FIG. 2 illustrates a process for receiving packet data in the receiver employing the general HARQ technique. In particular, FIG. 2 illustrates a process for receiving packet data in the case where the receiver sends the retransmission request message only once regardless of the channel environment upon detecting an error from the initial packet data. In the packet data receiving method show in FIG. 2, the receiver sends the retransmission request message in the same method regardless of the channel environment, i.e., regardless of a probability that an error will occur during retransmission of the packet data.
Referring to FIG. 2, upon receipt of initial packet data in step 201, the receiver determines in step 202 whether a transmission error has occurred in the received initial packet data. If the initial packet data has an error, the receiver proceeds to step 204. Otherwise, if the initial packet data has no error, the receiver proceeds to step 203. In step 203, the receiver transmits the error-free initial packet data to an upper layer and then ends the process. In step 204, the receiver transmits to the transmitter a retransmission request message NAK including the packet ID information including a version number and a sequence number in order to request retransmission of the initial packet data having an error. Thereafter, in step 205, the receiver receives the packet data retransmitted from the transmitter in response to the retransmission request message NAK, In step 206, the receiver determines again whether an error has occurs in the received retransmitted packet data. If an error is detected from the received retransmitted packet data, the receiver returns to step 204 and then performs again the above-stated operation Otherwise, if no error is detected from the received retransmitted packet data, the receiver provides the received retransmitted packet data to the upper layer in step 203 and then ends the process.
FIG. 3 illustrates a packet data combining process in the receiver employing the general HARQ technique. In particular, FIG. 3 illustrates a process for performing decoding by simply (or unconditionally) combining the received retransmitted packet data with the first received packet data regardless of the channel environment in the receiver employing the general HARQ technique. In the packet data combining process shown in FIG. 3, the receiver simply combines the received retransmitted packet data with the initial packet data, with no thought of the possibility that the received retransmitted packet data may have a bad influence on the decoding operation. This process is applied only to HARQ Type II and HARQ Type III, but not applied to HARQ Type I, which does not support the data combining technique.
In general, the HARQ scheme is divided into HARQ Type I, HARQ Type II and HARQ Type III. The HARQ Type I uses a constant redundancy, i.e., a constant data rate in both the initial transmission process and the retransmission process. The transmitter combines transmission data with a CRC (Cyclic Redundancy Check) code for error connection and then encodes the CRC-combined transmission data through channel encoding. Further, the transmitter transmits the encoded data through an assigned channel. The receiver then acquires the original data and the CRC in the reverse operation of the transmitter. The receiver transmits a response signal ACK or NAK to the transmitter according to the CRC check results. If no error is detected from the initial packet data, the receiver provides the received initial data to the upper layer, However, upon detecting an error, the receiver sends a retransmission request message NAK to the transmitter. The transmitter then retransmits the previously transmitted encoded data block upon receipt of the retransmission request message NAK.
However, the HARQ Type I has the following disadvantages. First, the HARQ Type I has higher throughput, compared with a pure ARQ scheme. However, as a signal-to-noise ratio (S/N) of a signal is increased more and more, the throughput becomes saturated to a code rate R of the FEC code, thus resulting in a reduction in the throughput ay compared with the pure ARQ. That is, the throughput cannot approach 1.0 (100%), but is saturated to the code rate R (<1.0) even at a very high S/N.
Second, the HARQ Type I improves the throughput by performing error correction using the FEC code, compared with the pure ARQ. However, since the HARQ Type I uses a constant redundancy, i.e., constant code rate regardless of variation in S/N. It has low transmission efficiency, Therefore, the HARQ Type I cannot adaptively cope with variations in the channel environment, thus causing a decrease in the data rate.
To solve the problems of the HARQ Type I the HARQ Type II and the HARQ Type III are used. The HARQ Type II and the HARQ Type III have an adaptive structure that adaptively determines an amount of redundancies used for the FEC code according to the channel environment. Therefore, the HARQ type II and the HARQ Type III have improved throughput compared with the HARQ Type I. That is, the adaptive structure reduces the amount of redundancies to a minimum, so that as the S/N of the signal is increased more and more, the code rate R of the FEC code approaches 1, thereby enabling the throughput to approach to 1. However, the adaptive structure increases the amount of redundancies or repeats the redundancies as much as possible, so that if the S/N of the signal is decreased, the code rate R of the FEC code approaches 0, thereby enabling the throughput not to approach to 0. Accordingly, the HARQ Type II and the HARQ Type III have improved throughput at both a low S/N and a high S/N.
A difference between the HARQ Type II and the HARQ Type III will be described below.
The HARQ type a transmits a data block with a code rate R1 set to a value equal to or less than ‘1’ during initial transmission, and transmits only the redundancies whose code rate is less than ‘1’ during retransmissions. Therefore, it is not possible to perform decoding with only the secondarily and thirdly transmitted redundancies, so that it is necessary to perform decoding by combining them with the previously transmitted data block (or redundancies).
On the other hand, the HARQ Type III transmits information on the data block even for the secondarily and thirdly transmitted redundancies using a complementary code. The HARQ Type III makes up for shortcoming of the HARQ Type II by making it possible to perform decoding during every transmission. In general, however, the HARQ Type III has lower throughput compared with the HARQ Type II in the good channel environment
With regard to the HARQ scheme, the W-CDMA mobile communication system has the following disadvantages, First, upon detesting an error from the initial packet data, the receiver transmits the version number and the sequence number of the retransmission-requested packet data, thus causing a decrease in efficiency of the retransmission request. That is, even in the worst channel environment the receiver sends the retransmission request message only once, with no thought of the possibility that an error may occur again in the packet data to be retransmitted.
Second, in the process of decoding the received retransmitted packet data, the receiver unconditionally combines the first received packet data with the received retransmitted packet data regardless of the channel environment during retransmission so that the received retransmitted packet data may have a bad influence on the decoding operation.